dt-bindings: stm32: add support of STM32MP157

This patch adds STM32MP157 SoC bindings.
Signed-off-by: NLudovic Barre <ludovic.barre@st.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAlexandre Torgue <alexandre.torgue@st.com>

Documentation/devicetree/bindings/arm/stm32.txt

@@ -7,3 +7,4 @@ using one of the following compatible strings:
   st,stm32f469
   st,stm32f746
   st,stm32h743
+  st,stm32mp157